This is a new RO1 entitled Neurodegeneration in aged SIV-infected primates that is a direct extension of our previously investigations in the pathogenesis of SIV encephalitis and neurodegeneration in aged non-human primates. Use of combined active antiretrovirals (CART) in developed countries has led to a near disappearance of severe encephalitis. Unfortunately chronic HIV infection continues to exact a toll on the nervous system with increased prevalence of a spectrum of neurocognitive and motor dysfunctions termed HIV-associated neurocognitive disorders (HAND). CART has also permitted people to survive longer with HIV infection and the CDC projects that by 2015 over half of HIV infected individuals in the US will be over the age of 50. Coupled with the aging process, the extended exposure to both HIV and antiretroviral drugs appears to increase their risk of neurologic and neuropsychiatric complications. In this application we propose to use a well-established non-human primate (NHP) model of chronic lentiviral infection, SIV infection of Macaca mulatta (Rhesus Macaque RM), to model HAND in aged macaques (>20 years old), map the neurological signs to behavioral abnormalities and begin to elucidate the neurological and immunological pathogenesis of this debilitating disease. While no animal disease model is perfect, numerous similarities between simian and human nervous systems, between SIV and HIV infection and the capacity to manipulate and monitor central nervous system (CNS) damage, make the macaque model optimal for these studies. Using 5 groups of SIV infected and control RMs, we will assess the presence of neurocognitive abnormalities and the role of viral suppression in exacerbating age related neurodegeneration. Findings from this grant will have immediate implications on the treatment of aged HIV infected humans. Building upon our previous decade of experience with SIV infection of macaques as a model of chronic HIV infection and our recent studies of neuroinflammation in aged NHPs (Kofler et al 2011 (41) see appendix), we will test our overarching hypothesis that: chronic systemic CART with or without chronic lentiviral infection exacerbates age related damage to the nervous system principally through CNS stress and associated innate immune activation. We will assess whether aggressive suppression of systemic lentiviral infection with CART exacerbates age related cognitive abnormalities and beta amyloid related neuropathology (e.g. plaques, tangles, abnormal tau phosphorylation or beta amyloid oligomeres). Alternatively, HAND may persist even in the presence of viral suppression or as a result of CART alone. In our second specific aim we will use state of the art quantitative neuropathological analysis to elucidate the pathological substrate of HAND. Knowledge of the pathogenesis of neurological dysfunction in the simian model will help define pathways for intervention to mitigate the human disease.